This invention relates generally to the field of disc drive storage devices, and more particularly, but not by way of limitation, to improvements in the detection of angular position of a magnetic recording disc.
Disc drives are commonly used as the primary data storage and retrieval devices in modern computer systems. In a typical disc drive, user data are magnetically stored on one or more discs that are rotated at a constant high speed and accessed by a rotary actuator assembly having a plurality of read/write heads that fly adjacent the surfaces of the discs. A read channel and interface circuit are provided to recover previously stored data from the discs to the host computer.
A closed loop digital servo system such as disclosed in U.S. Pat. No. 5,262,907 issued Nov. 16, 1993 to Duffy et al., assigned to the assignee of the present invention, is typically used to control the position of the heads relative to tracks on the discs. The tracks are defined from servo data written to the surfaces of the discs during manufacturing. The servo system of a disc drive utilizes the servo data to maintain the selected head over a particular track so that the user data can be transferred to and from user data fields defined on the tracks. The servo system further uses the servo data to control the movement of the head from one track to another during a seek operation.
As will be recognized, modern disc drives typically employ an embedded servo scheme wherein the servo data are arranged in angularly spaced servo fields which extend as a plurality of servo wedges across the disc surfaces, like spokes of a wheel. The user data fields (or xe2x80x9csectorsxe2x80x9d) are subsequently defined on the tracks between successively occurring servo fields during a formatting operation. As the sampling rate of the servo data is typically insufficient to maintain the heads within predetermined off track boundaries, the servo system uses a multi-rate observer to provide estimates of head position, velocity and bias at times when the heads are disposed over the user data fields. Such observers (or xe2x80x9cestimatorsxe2x80x9d) are well known in the art and are discussed, for example, in U.S. Pat. No. 5,585,976 issued Dec. 17, 1996 to Pham, assigned to the assignee of the present invention.
Each servo field typically includes an automatic gain control (AGC) field with an oscillating pattern that tunes the servo system for receipt of the remaining servo data in the servo field. An index field indicates angular position of the disc. A track address field indicates radial position on the disc by storing a unique track address sequence for each track (typically expressed in Gray code). A position field provides a plurality of servo burst patterns which, when transduced by the head, provide burst signals that can be combined to provide a position error signal (PES) indicative of intra-track position.
Of particular interest is the index field which enables the servo system to determine the location of the xe2x80x9cstarting pointxe2x80x9d on the track as the track rotates underneath the head. This starting point is referred to as an xe2x80x9cindex reference positionxe2x80x9d and can be thought of as representing an angular position of zero degrees (0xc2x0) on the circular disc. It is desirable to restrict as far as practicable the amount of disc surface area needed by the servo data in order to increase the surface area available to store user data (to achieve greater data storage capacity). For reference, the ratio of area occupied by servo data as compared to the total amount of disc surface area used to store both servo data and user data is sometimes referred to as xe2x80x9cservo overhead,xe2x80x9d and typically ranges from 6-8%.
Thus, to reduce servo overhead, index fields have typically been made as small as practicable and have been provided with one of two different magnetically stored patterns: a xe2x80x9cnon-indexxe2x80x9d pattern which is used for all of the index fields on the track that do not correspond to the index reference position (i.e., are not at the 0xc2x0 position) and an xe2x80x9cindexxe2x80x9d pattern which is used to identify the servo field that corresponds to the index reference position (i.e., the one field at the 0xc2x0 position). If the non-index field pattern is characterized as a logical zero (0), and the index field is characterized as a logical one (1), then the servo system will xe2x80x9clookxe2x80x9d for the logical one (1) pattern as the successive index field portions of the servo fields on a particular track pass under the head.
While using relatively short, binary state index field patterns advantageously allows greater user data storage capacity, a problem can arise if an anomalous condition (such as a grown thermal asperity) prevents the servo system from detecting the index reference position. If angular position information is not otherwise available, the disc drive may be unable to retrieve previously stored user data on the affected track. To address this problem, prior art methodologies have introduced the use of non-binary state index field patterns, such as patterns with an informational content similar to the track address field, that provide an indication of the particular angular location of each index field. However, the additional information content of such patterns undesirably require a lengthening of each index field, increasing servo overhead and reducing the available data storage capacity of the disc drive.
Accordingly, there is a need for improvements in the art to enable disc drives to detect index reference position in a disc drive without increasing the amount of servo overhead. It is to such improvements that the present invention is directed.
The present invention is directed to an apparatus and method for improving detection of index reference position in a disc drive.
In accordance with preferred embodiments, a disc includes servo fields used by a servo circuit to control position of a head adjacent the disc and data fields used to store user data, with at least one data field between each successive pair of servo fields. Each servo field includes a binary index field used to indicate angular position on the disc.
A multi-bit index detection word is selected having at least two bits at a first logical state (such as logical 0) and at least two bits at a second logical state (such as logical 1). A plurality of index fields leading up to, and including a selected index field at the index reference position, are written with respective patterns corresponding to the logical sequence of the index detection word. As the disc rotates, the servo circuit reads each of the successive index fields and identifies the index reference position upon detection of the index detection word.
By using multiple, successive binary state index fields to identify the index reference position, increased robustness is obtained without the requirement for additional servo overhead.
These and various other features as well as advantages which characterize the present invention will be apparent from a reading of the following detailed description and a review of the associated drawings.